The primary objective of the Radiological Physics Center (RPC) is to assure NCI and the cooperative clinical trial groups that participating institutions have adequate quality assurance (QA) procedures and no major systematic dosimetry discrepancies, so they can be expected to deliver radiation treatments that are clinically comparable to other institutions in the cooperative group. In order to accomplish this objective, the RPC monitors the basic treatment unit output and brachytherapy source strength, the dosimetry data used by the institution, the calculation algorithms used in treatment planning, and QA procedures. The methods of monitoring include on-site dosimetry review visits and various remote audit tools. During on-site reviews, key personnel are interviewed, physical measurements are made on therapy machines, dosimetry and QA data are reviewed, treatment planning algorithms are tested and patient dose calculations are evaluated. The remote audit tools include: 1) mailed TLD evaluated on a periodic basis to verify output calibration; 2) comparison of dosimetry data with RPC standard data to verify comparability of the dosimetry data; 3) evaluation of reference and/or actual patient calculations to verify the treatment planning algorithms; 4) review of an institution's written QA procedures and records; and 5) mailed anthropomorphic phantoms to verify tumor dose delivery for special treatment techniques. The RPC continuously modifies its techniques to reflect new protocols and changes in practice at participating institutions. The RPC currently monitors 1338 radiation therapy facilities. Any discrepancies found by the RPC are pursued to help the institution resolve them. Thus, the RPC's overall QA program impacts not only on clinical trial patients, but on the quality of all patients treated at the institution.